1. Field of the Invention
The present invention relates to an optical head used for optically recording/reproducing information, and more particularly to an optical head which is easy to adjust, requires only inexpensive parts, and offers reliable focus error detection and tracking error detection.
2. Description of Related Art
FIG. 26 illustrates a general construction of a prior art optical head reported by Shinoda and Kondo, "A small Write-Once Optical Head," Abstracts of Eleventh Optical Symposium, Jul. 1, 1986, pp. 30-40, sponsored by Oyo Butsuri Gakkai Kogaku Konwakai (Optics Division, The Japan Society of Applied Physics).
Referring to FIG. 26, a light source such as a semiconductor laser emits a beam of light for recording and reproducing information recorded on an optical disc.
A polarization beam splitter 2 passes the light E emitted from the light source 1 toward an information recording medium 6 and reflects the light reflected back from the recording medium 6 to a focus error detection system, tracking error detection system, and information recording/reproducing system, which will be described later. The light passing the polarization beam splitter 2 is collimated by a collimator lens 3 and then passes through a quarter wave plate 4. The light then passes through an objective lens 5 which in turn focuses the light onto the information recording medium 6 such as an optical disc. The information recording medium 6 is formed with information tracks 7 therein.
The polarization beam splitter 2 directs the light reflected back from the information recording medium 6 to a roof prism 8. The roof prism 8 branches the light into two beams of light each of which has a semicircular cross section. Disposed downstream of the roof prism 8 are a concave lens 9 and beam splitter 10 in this order. A portion of the light passes through the beam splitter 10 to a photodetector assembly 12 and the remaining portion of the light is reflected by the beam splitter 10 to a two-division photodetector 11.
The two-division photodetector 11 includes two light-receiving elements 11a and 11b which receive the light reflected by the beam splitter 10. The two-division photodetector 11 cooperates with a subtractor or differential amplifier 13 to form the tracking error detection system which detects the tracking error of the light spot on the information track 7.
The photodetector assembly 12 receives the light which is reflected from the information recording medium 6 and passes through the beam splitter 10. The photodetector assembly 12 includes four light-receiving elements 19-22, and cooperates with differential amplifiers 15, 16, and 17 and the roof prism 8 to form the focus error detection system which detects the focus error of the light spot. The differential amplifier 15 and 16 outputs the difference of the outputs from the light-receiving elements 22 and 21 and the difference of the outputs from the light receiving elements 20 and 19 respectively. The differential amplifier 17 outputs the difference between the outputs of the differential amplifiers 15 and 16.
The two-division photodetector 11 receives the light reflected by the beam splitter 10. The two outputs of the two-division photodetector 11 are directed to the differential amplifier 13 which outputs the difference between the two outputs, and to an adder 14 which outputs the sum.
Then, the light is reflected by the information recording medium 6 back through the objective Lens 5, quarter wave plate 4, and collimator lens 3. The reflected light beam is then incident upon the polarization beam splitter 2. The reflected light has rotated by 90.degree. since it passes through the quarter wave plate 4 twice, once directly and again after being reflected by the medium 6. The reflected light beam is therefore reflected by the polarization beam splitter 2. Then the light emanating the polarization beam splitter 2 travels through the roof prism 8, concave lens 9 and is then incident upon the beam splitter 10. The splitter 10 separates the incident light into two light beams: a Light beam traveling toward the two-division photodetector 11 and a light beam traveling toward the photodetector assembly 12. The two-division photodetector 11 detects a tracking error signal TES by the push-pull method white the roof prism 8 and photodetector assembly 12 detect a focus error signal FES by the pupil obscuration method. The tracking error signal TES is amplified by the differential amplifier 13 and drives a tracking actuator of an objective lens driving mechanism. The focus error signal FES is amplified by the differential amplifiers 15, 16 and 17 and drives a focusing actuator of the objective lens driving mechanism.
The operation of the focus error detection of the optical head shown in FIG. 25 will be described with reference to FIGS. 26 and 27.
FIG. 27 illustrates a case where the light beam E is exactly focused on the information recording medium 6. Referring to FIG. 26, a lens 18 is equivalent to a combination of the collimator lens 3 and the concave lens 9 shown in FIG. 25 and therefore operates in the same way as the optical system that includes the collimator lens 3 and concave lens 9. The two-division photodetector 12a is positioned relative to the prism 8 such that when the light beam E is exactly focused on the recording medium 6, a light beam R1 is focused on the two-division photodetector 12a with the light spot centered on the boundary between the light-receiving elements 19 and 20.
The two-division photodetector 12b is positioned relative to the prism 8 such that when the light beam E is exactly focused on the information-recorded surface of the recording medium 6, the light beam R2 is focused on the two-division photodetector 12b with the light spot centered on the boundary between the light-receiving elements 21 and 22.
Therefore, when the beam E is focused on the information recording surface of the information recording medium 6, the light-receiving elements 19 and 20 receive the same amount of light and therefore the outputs of the light-receiving elements 21 and 22 also receive the same amount of light. Therefore, the sum S1 of the outputs of the light-receiving elements 19 and 22 is equal to the sum S2 of the outputs of the light-receiving elements 20 and 21.
FIG. 28 is a graph illustrating the relation between the focus error .DELTA.z, i.e., the distance between the converged spot and the information recording surface of the information recording medium 6. The focus error signal FES varies in proportion to the focus error .DELTA.z over a range called linear zone. The linear zone when the pupil obscuration method is used is in the range of 2-3 .mu.m for NA of 0.5-0.6 of the objective lens. Reference is made to the following documents regarding to details of linear zone: G. Bouwhuis et al., Principles of Optical Disc System," published by Adam Hilger, pp. 77-79 (1985) and Irie et al., "Focus Sensing Characteristics of the Pupil Obscuration Method for Continuously Grooved Disk," Japan Journal of Applied Physics, vol. 26, pp. 183-186 (1987)."
The thus obtained focus error signal FES is directed via a phase compensator/amplifier to an objective lens-driving mechanism, not shown, which drives the objective lens so as to maintain the light E focused on the information recording surface.
The roof prism 8 is oriented so that its roof edge extends in a direction substantially perpendicular to the x-direction which is tangent to the information track 7 in the medium 6. The orientation of the roof prism 8 minimizes disturbances which may come into the focus error signal FES when the converged spot moves transversely of the information tracks 7 formed in the information recording medium 6.
Reference is made to the aforementioned Irie et al. reference regarding details of the minimizing of disturbances.
The adjustment of the focus error detection system of the optical head shown in FIG. 26 will be described.
The focus error detection system drives the lens 9 to move back and forth on its optical axis (z-direction), and to move a package 24 in which the two-division photodetectors 12a and 12b are housed. The package 24 is moved in the x-y plane in which the two-division photodetectors 12a and 12b are disposed, so that when the light E is focused on the information-recording surface of the medium 6, the light spots on the corresponding two-division photodetectors 12a and 12b are the smallest in size and are centered on the boundary between the two light-receiving elements 19 and 20 and on the boundary between the two light-receiving elements 21 and 22, respectively.
FIG. 29 illustrates the light spots on the two-division photodetectors 12a and 12b before adjustment of the optical head, the two light spots not being the smallest even though the light E is focused on the information-recording surface of the medium 6.
Then, the concave lens 9 is moved back and forth in the direction of its optical axis to a position where the sizes of the converged spots 25 and 26 are minimum.
Then, the package 24 is moved in the x-direction to bring one of the converged spots on the division line. FIG. 30 shows the converged spot 26 brought on the division line. Thereafter, the package 24 is moved in the x-y plane so as to bring another converged spot 25 on the division line as shown in FIG. 31. It is not easy to simultaneously bring both the converged spots 25 and 26 on their corresponding division lines since the converged spot 26 deviates from its corresponding division line as the converged spot 25 is moved on the tight receiving surface of the light-receiving elements 19 and 20. The package 24 is slowly moved in the x-y plane while monitoring the output signals of the two-division photodetectors 12a and 12b that represent the positions of the converged spots 25 and 26 relative to their corresponding division lines, respectively.
Finally, the adjustment of the tracking error detection system of the optical head will be described.
The tracking error detection system causes the light-receiving elements 11a and 11b of the two-division photodetector 11 to move in order to adjust the positions of the light receiving elements so that the elements 11a and 11b receive the same amount of light as shown in FIG. 32 when the converged spot of the light E is focused on the information recording surface. With the prior art optical head shown in FIG. 26, the two-division photodetector 11 may be at any position in the converged light beam along the optical axis of the light beam except at the focal point. The size of the light spot incident upon the two-division photodetector 11 is, for example, in the range of 0.5-0.7 mm diameter if the light receiving element has a light-receiving area of 1 mm.times.1 mm.
The prior art optical head of the aforementioned construction is disadvantageous in that the focus error detection and tracking error detection each need an exclusively provided photodetector.
During the focus error detection, the package 24 is moved in the x-y plane so as to bring the converged spots 25 and 26 on the corresponding division lines, respectively. Bringing one of the spots centered exactly on its corresponding division line results in some deviation of the other. In other words, the two converged spots 25 and 26 cannot be positioned independently from each other. Therefore, another disadvantage of the prior art optical head is that the adjustment of the focus error detection system is a time consuming operation. In addition, the focus error detection and tracking error detection system must be adjusted individually.
Still another problem with the prior art optical head is that the focus error signal FES linearly varies only in a narrow range of about 2-3 .mu.m referred to as linear zone. A narrow linear zone is disadvantageous in that the servo control is susceptible to external disturbances such as mechanical impacts and vibrations, being difficult to maintain the light E focused on the information recording surface of the medium 6. A minute positional deviation of the photodetector assembly 12 results in a large change in the focus error signal FES and the focus error directly reflects the minute change in electrical offset within the servo circuit.